The use of high power variable-frequency drive has become increasingly popular with the rapid development of modern power electronic technology and microelectronic technology. In particular, under the circumstance that the operation reliability of medium-voltage/high-voltage devices is not high enough while the cost thereof is expensive, such a problem has been well resolved in recent years by means of cascading low-voltage power cells. Therefore, the application fields and range of cascaded H-Bridge medium voltage drive have become more and more broad, which enables an efficient and proper use of energy (especially the electric energy).
In cascaded H-Bridge medium voltage drive, the number of power cells varies with different voltage levels of the drive system. Taking a drive system of 6 kV as an example, there are generally 5 or 6 power cells connected in series for each phase, in which each power cell provides the same output voltage, and thus there are a total of 15 or 18 power cells. As to a drive system of 10 kV, the number of power cells even reaches as many as 27. With so many power cells, once a malfunction occurs in a certain power cell, the whole drive system may fail and be unable to keep operating if there isn't any proper solution, which may result in an economic loss or an aggravation of accident. Increasing of the number of power cells makes the drive system less reliable.
This problem can be resolved effectively by a bypass circuit of a power cell. A major circuit turns on when a power cell operates normally, that is, the whole drive system can operate normally as long as the major circuit operates normally. Therefore, the bypass circuit of the power cell in cascaded H-Bridge medium voltage drive is such a bypass device that makes a major circuit of the power cell bypassed (or shorted) when a failure occurs in the major circuit of the power cell, so as to prevent the whole drive system from shutting down. In related arts, there is one bypass circuit for each power cell in the cascaded H-Bridge medium voltage drive. When the major circuit of a certain power cell fails and thus is unable to output a voltage normally, in order to avoid the whole cascaded H-Bridge medium voltage drive from shutting down, the bypass circuit of failed power cell bypasses the corresponding main circuit, that is, the corresponding bypass circuit will be switched on for operation, thereby the major circuit of failed power cell is bypassed (shorted), thus the whole cascaded H-Bridge medium voltage drive is enabled to keep operating, which ensures that no economic or other kinds of loss occurs due to shutting down of the cascaded H-Bridge medium voltage drive.
Chinese Patent Application No. 201010218945.2 discloses a unit bypass circuit of unit cascaded H-Bridge medium voltage drive, which is inefficient since a mechanical-operated switch blade therein needs to be closed manually when the power cell malfunctions, and then the mechanical-operated switch blade needs to be opened after the malfunctioned power cell is replaced.
There is also disclosed another technical solution in related arts, in which a mechanical means of contactor is adopted as a bypass circuit as shown in FIG. 1. This bypass circuit, of which the close and open are controlled by an auxiliary switch KM, is of a high reliability and has its advantages in low power cascaded H-Bridge medium voltage drive system. However, it will be more complicated and relative expensive for the system design in high power variable-frequency drive due to a large size and a high cost of the contactor.
In summary, the bypass circuit in the cascaded H-Bridge medium voltage drive in related arts has disadvantages such as being expensive and large-sized, hence needs to be further improved.